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Search: id:A087979
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| A087979 |
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a(n) = Min {x : sigma[x]=phi[nx], x is not a prime}, least nonprime solutions to sigma[x]=phi[nx]. |
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3
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| 1, 1, 15, 14, 14, 6, 6, 42, 30, 42, 168, 210, 210, 420, 840, 20790, 20790, 9240, 9240, 83160, 120120, 3984120, 5165160, 43825320, 26860680
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OFFSET
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1,3
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EXAMPLE
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n=4: a(4)=14, sigma[14]=24=phi[4.14]=phi[56],
n=5: a(5)=14, sigma[14]=24=phi[5.14]=phi[70].
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MATHEMATICA
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ds[x_, de_] := DivisorSigma[1, x]-EulerPhi[de*x] a[n_] := Block[{m=1, s=ds[m, n]}, While[(s !=0||PrimeQ[m])&&!Greater[m, 4000000], m++ ]; m]; Table[a[n], {n, 22}]
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CROSSREFS
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Cf. A074891, A000203, A000010, A088830.
Sequence in context: A023457 A004456 A055126 this_sequence A048294 A069785 A010854
Adjacent sequences: A087976 A087977 A087978 this_sequence A087980 A087981 A087982
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KEYWORD
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nonn
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AUTHOR
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Labos E. (labos(AT)ana.sote.hu), Sep 29 2003
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EXTENSIONS
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More terms from David Wasserman (wasserma(AT)spawar.navy.mil), Jun 20 2005
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